1. Field of the Invention
The present invention relates to a method and apparatus for evaluating formation and surface characteristics of a fibrous sheet material and predicting its performance in finished articles. In particular, the invention relates to the effects of formation and surface smoothness on the printability of papers.
2. Description of the Prior Art
Evaluating papers manufactured by printing to judge how they are likely to perform in printing is difficult today because of a lack of methods and equipment which reflect what actually happens to papers in printing processes. Those instruments which do reflect printing conditions are too slow or comprise expensive equipment or require skilled persons to operate. What is needed is a printability tester that is fast and simple enough to be used in the mill environment, yet produces results that relate to qualities important in printing.
There are printability testers for evaluating papers which are basically printing presses. Their advantage is that they duplicate the pressure conditions to which a paper is subjected in printing. Conventional printing process such as letterpress, offset, and gravure require physical contact between ink bearing surfaces and the paper being printed. Contact pressures vary depending upon the method employed, but are generally on the order of tens of kilograms per square centimeter. Early workers with printing testers had difficulty in eliminating print differences resulting from an inability to control the amount of ink applied to a test paper. For example, applying too much ink to a printing plate causes difficulties because the ink tends to flow from contact areas into non-contact areas due to wetting forces. Reducing the amount of ink used quickly reaches a limit as the inks become very difficult to transfer uniformly. The result in either case is that quality differences related to the papers can become veiled by the difficulties related to ink transfer.
Some printability tester manufacturers eliminated inking difficulties by selecting the rotogravure printing process. In gravure printing testers, an image is formed on a printing plate by mechanically engraving, or chemically etching a multiplicity of minute dots or cells (30-120 microns in diameter) on a cylindrical printing plate. In printing, each cell is filled with ink and the excess wiped or doctored from the plate. The depth of the cells and their spacing determine the tone produced on the paper.
If a paper fails to contact a cell, no ink will transfer and there will be a skip or missing dot on the resulting print. It is evident that print quality can be judged by the number of missing dots. The gravure process eliminates the non-uniformity of ink transfer problem. Where the gravure tester is otherwise designed to ensure that the only test variable remaining is the paper itself, missing dot count is a measure of gravure printability of a given paper. It has been found that this measure of gravure quality correlates well with performance of papers in other printing processes.
The problem with printability testers based upon gravure printing is that presently available testers are precision printing presses. These testers are large for testing instruments and, while perhaps suitable as a laboratory instrument, are not suited for paper mill use by paper technicians having little printing experience. These press testers are also expensive and therefore not many mills are able to afford and maintain one.
Research to determine which fundamental qualities of paper affect printing quality suggests that something less than a printing press may be used to predict printing quality. In examining gravure prints, it was discovered that most missing dots occur on papers in local areas of low fiber density. See J. Marton, GRI Report No. M-102 (1978). Fiber density distribution within a paper's thickness over its surface area or "formation" is a well-known characteristic used by papermakers to judge the quality of their product. Formation is commonly defined as the point-to-point distribution of the physical density of the fibers (in terms of weight-per-unit volume or basis weight) of a paper sheet over its area.
In relating Marton's missing dot observation noted above to formation, analysis shows that when a uniform pressure is applied to the surface of a paper, only the least compressible (the most dense) elements of the paper structure will support the load. Compressible (or less dense) elements will give in and provide just a fraction of the total resistivity of the paper to the pressure.
Other researchers have recognized that intrinsic formation of a sheet is an important, if not the most important, property of a paper. O. Kallmes in "New Approach to Measuring Formation--A Matter of Uniformity," Paper Trade Journal,pp. 46-50 (Oct. 25, 1971) in examining a number of techniques and devices for measuring formation, noted the impact on printing of highly localized microscale variations. Kallmes concluded that the fundamental printability quality characteristic is "uniformity" of formation. He found that gravure printing quality is extremely sensitive to "uniformity" of formation, although both variation in caliper and surface smoothness contribute.
The known devices for measuring formation and relating it to printing generally measure variations in transmittance of radiation through a paper sample. They differ with respect to the type of radiation used (visible light, X-rays or beta rays) and the techniques used to transform transmittance variations into an index of formation.
In one instrument a test paper is sandwiched between a beta ray emitting film and a photographic film plate. After exposure of the film there is a negative picture of the mass distribution of the fibers over the surface examined. The exposed film is scanned by an optical microdensitometer. These results are analyzed by computer producing a basis weight histogram and various other statistics. A difficulty with this technique, which is widely recognized as producing a true image of the formation of a paper throughout its thickness, is that it is complex, costly, requires a highly trained operator and is extremely time consuming.
A number of instruments are based on measurement of transmission of a highly collimated beam of light through a paper sample. The transmitted signal is analyzed to produce a basis-weight contour map in which the higher the basis weight the heavier the shading. However, sheet opacity variations caused by surface fillers and the like interfere with results.
Another approach in quantifying the printability of papers regards surface smoothness of a paper as a primary indicator of the perfection of contact between ink and paper. These testers, commonly called smoothness testers, are often based on the measurement of the rate at which air, under a given pressure, can leak between the surface of the paper and a hard, smooth surface pressed against it. The lower the rate of leakage, the smoother the surface. A number of other smoothness testers utilize optics. Typically, the sample is pressed against a glass prism. A parallel beam of light is passed through the prism normal to the surface of the paper. Reflected light at certain angles is measured at photoelectric cells to determine the percentage of the paper surface which actually touches the prism. The percentage is by definition of this test a measure of the printing quality of the paper.
The difficulty with the radiation transmittance formation testers and the optical and air leakage smoothness instruments is that they do not duplicate the printing process conditions under which ink transfers. A principal defect is that pressures utilized, if any, are insufficient to account for the effects of typical printing pressures. A defect of instruments that do use pressure, such as air leak devices, is that they tend to average out surface voids. In printing, however, it is the individual voids which produce missing dot printing defects.
Those instruments which are correlatable with printing all require a great deal of skill to operate to produce consistent results. None of the instruments available today are truly suitable for use in the mill by paper lab technicians.
As a consequence today, while papermakers recognize that formation is a fundamental quality of their product, they are unable to find a testing apparatus and procedure which is suitable for the mill environment with respect to either cost, timeliness or reproducibility. The present tests used in the mill tend to be purely subjective. In one typical test, for example, a specimen is viewed on a light box and its highlighted pattern is compared with control papers. This test is, of course, difficult to reproduce or replicate between testing technicians.